1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus which obtains a color image output for example by fixing toner images overlaid according to monochrome color images corresponding to color-separated color components to a transferred material.
2. Description of the Related Art
In an electrophotographic color image forming apparatus, a certain surface potential is given to a photoconductor capable of holding an electrostatic latent image, the photoconductor surface potential according to a background or image part is changed selectively, a toner image is obtained by supplying a developing agent (toner) to that part, and the toner image is transferred to an output medium (transferred material).
Nowadays, user needs have diversified, and it has been demanded to output a color image with high quality and exact color reproducibility to various media including a paper sheet of 50-250 g/m2, transparent resin sheet, and adhesive coated stickers.
A color image forming apparatus includes a black developing unit which outputs a black or Bk image, and a color developing unit which outputs three monochromatic color images of C (cyan), M (magenta) and Y (yellow) forming a color image.
Toner images of four colors formed by respective developing units are sequentially laid on a photoconductor or transfer material, that is, an ordinary paper or OHP sheet, and fixed to the transfer material by a fixing unit.
A method of increasing a fixing temperature or decreasing a fixing speed when fixing a color image with multiple toner images overlaid to a transfer material has been proposed to ensure high color reproducibility and fixing rate even for thick paper sheets and OHP sheets.
For example, Jpn. Pat. Appln. KOKAI Publication No. 11-2939 proposes an image forming apparatus which decreases the linear velocity of an intermediate transfer belt to be lower than that for non-thick paper sheets, when transferring an image to a thick paper sheet in a secondary transfer process, and feeds a transfer paper sheet on the basis that the reference mark of the intermediate transfer belt is detected first.
In the apparatus disclosed by the Jpn. Pat. Appln. KOKAI Publication No. 11-2939, the timing for feeding a transfer paper sheet is set by using the reference mark of an intermediate transfer belt in a secondary transfer process. Thus, the timing can be obtained only by rounds of rotation of the intermediate transfer belt.
Therefore, when the circumference of an intermediate transfer belt is long, or when an image can be formed on a transfer paper sheet with a wide area, the image forming is delayed by the time equivalent to one round of rotation of the intermediate transfer belt when forming an image in a secondary transfer process where the image transferred to the intermediate transfer belt is transferred to a transfer paper sheet. Further, when forming an image of corresponding size on a transfer paper sheet with a small area (maximum length) compared with the circumference length of the intermediate transfer belt, there arises a problem that the time required to the secondary transfer of image to a transfer paper sheet after the first transfer of the image to the intermediate transfer belt is increased despite the size (length) of the transfer paper sheet being small.